Commercial aromatic polyesters, hereinafter "polyarylates", are derived from bisphenols, typically bisphenol A, and aromatic dicarboxylic acids, typically isophthalic acid which may be admixed with up to 80% of terephthalic acid. Commercial polyarylates are rigid, amorphous polymers which exhibit high heat distortion temperatures. These properties are advantageous in many applications including automobile parts for under-the-hood use and double-walled housings for appliances. There are many components of automobiles and other systems which are hollow and are most readily manufactured by blow molding. In contrast to commercial grades of polyester resins, such as poly(ethylene terephthalate), there are some commercial grades of polyarylates that can be blow molded. Because of the amorphous character of the polyarylates and the limited sensitivity of their viscosity to temperature changes, they have what might be called a relatively wide "temperature-window" where processing by blow molding is practical. Ordinary polyesters; e.g., ones made from aliphatic alcohols, have virtually no temperature-window for blow molding because of the low viscosity of their melts coupled with the fact that crystallization is encountered before lowering of the temperature raises their viscosity sufficiently for blow molding.
For applications where the high maximum heat distortion temperature property of polyarylates is not of importance, it is advantageous, particularly from a cost standpoint, to admix a lower-cost polyester resin with a polyarylate. Such blends retain the blow moldable characteristic of the polyarylates when blending of the melt of the two polymers is continued for a sufficient period of time such that the resulting blend is amorphous. If blending is conducted for only a short period of time, portions of the resulting blend will crystallize and interfere with attempts to blow mold the product.
It has been found that the addition of small amounts of ethylene copolymers containing epoxy groups to polyarylates or amorphous blends of polyarylates with polyesters increases their melt viscosity. This effect widens the temperature-window for processing by blow molding. While the polyarylate compositions modified with the epoxy-containing copolymers are readily blow molded even into articles having complex cross-sections, it has been found that the wall thickness of objects having complex cross-sections may vary by more than 100% when the thickest portions of the article are compared to the thinnest portions of the article. This irregularity in wall thickness is a serious drawback to the use of polyarylates because the only way to obtain a predetermined minimum wall thickness throughout an entire blow molded article is to increase the amount of polyarylate used to make the molding. This solution to the problem is costly in terms of material, weight and processing time. Thus, there is still a need for improved polyarylate-based blow molding compositions capable of providing complex moldings having uniform wall thickness throughout. Such compositions would provide blow molded objects having maximum strength for minimum weight and cost.